As is known, many varieties of memory storage devices (e.g. disk drives), such as magnetic disk drives are used to provide data storage for a host device, either directly, or through a network such as a storage area network (SAN) or network attached storage (NAS). Typical host devices include stand alone computer systems such as desktop and laptop computers, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disks (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices such as video game systems and digital video recorders. These devices provide high storage capacity in a cost effective manner.
The structure and operation of hard disk drives is generally known. Hard disk drives (HDDs) include, generally, a case, a hard disk having magnetically alterable properties, and a read/write mechanism including Read/Write (RW) heads operable to write data to the hard disk by locally alerting the magnetic properties of the hard disk and to read data from the hard disk by reading local magnetic properties of the hard disk. The hard disk may include multiple platters, each platter being a planar disk. The read/write mechanism also includes a disk drive controller.
All information stored on the hard disk is recorded in tracks, which are concentric circles organized on the surface of a platter. Data stored on the disks may be accessed by moving RW heads radially as driven by a head actuator to the radial location of the track containing the data. To efficiently and quickly access this data, fine control of RW hard positioning and sampling is required. The track-based organization of data on the hard disk(s) allows for easy access to any part of the disk, which is why hard disk drives are called “random access” storage devices.
Since each track typically holds many thousands of bytes of data, the tracks are further divided into smaller units called sectors. This reduces the amount of space wasted by small files. Each sector holds 512 bytes of user data, plus as many as a few dozen additional bytes used for internal drive control and for error detection and correction.
Within such HDDs, disk drive controllers control the various processes associated with the read/write of data to the physical media (hard disk). As the amount of data stored to the physical media increases, the ability to accurately read data from the physical media is adversely effected. This is further complicated as hard disk drives are forced into smaller form factor devices, the physical media or disk itself becomes smaller further increasing the need for increased storage density. Thus, smaller form factors and higher storage densities make these disks more susceptible to fluctuations of a disk clock as the disk itself may not necessarily rotate in circular fashion. The disk (i.e. physical media) itself may not necessarily rotate in a truly circular path. There may be some variations where the center of rotation may be off axis. Other variations arise from the mechanism used to drive the rotation of the disk, resulting in an elliptical path of the hard disk about the axis of rotation. This ellipse may change as the physical media shifts on the spindle or axis of rotation. These effects are most noticeable in small form factor device. In order to ensure that data is properly read from small form factor devices it is necessary to track the variations associated with the disk rotation to properly sample the data.